It is possible in principle to have a flat universe in which light can circle around. It is not known whether our Universe is such, and it probably never will be known. If our Universe does circle around in this way, it does so on a scale that is larger than we are able to observe.
Thank God. Not that I’m glad you are confused, but I’m glad I’m not the only one out here that has questions, and then more questions based on the answers. This is such a fascinating field of study to me, but I find a lot of it difficult to accept. Perhaps my logical brain has a hard time viewing answers in the abstract.
This actually clarified the picture a great deal for me. (Thank you, Obi-wan Kenobi!!)
But this leads me to another question. If everything is expanding away from everything else (using this analogy or the balloon analogy), how is it that galaxies collide? Andromeda is heading for the Milky Way and is going to intersect with it eventually. How does that happen if everything is flying away from everything else? Or, is the idea of everything moving away from everything else not accurate today? Since Andromeda and the Milky Way were formed after the big bang, they are not necessarily flying away from each other/
Exapno, thanks for the book suggestion. I remember hearing how Hawking’s book “A brief history of time” was easy for people to read and understand, and after reading it, feeling like a monkey. I hope this book is better for me.
Everything is moving apart only on the very large scale. At smaller scales, gravitational attraction overcomes the expansion. The Milky Way and Andromeda and a bunch of smaller galaxies are all bound together by gravity into The Local Group.
Space is expanding, meanwhile galaxies are still moving relative to each other. The Andromeda Galaxy is part of the Local Group consisting of around 50 galaxies (including our own, of course) and is about 2.5 million LYs away. It’s one the few blue-shifted galaxies, because despite the expansion of space itself, it’s the closest spiral galaxy to us, and cruising toward us faster than this expansion at about 70 miles a second, and will arrive in about 4.5 billion years, so don’t wait up.
So, that said, using Hubble’s Law, space seems to be expanding at around 45 miles per second per every million parsecs (A parsec is ~3.25 LYs, a LY is about 6 trillion miles,). So, with Andromeda being 2.5 Million LYs away the expansion of space itself should be pushing Andromeda away from us at around 34 miles a second. So, if due to gravity or inertia, or whatever circumstances, it’s still headed toward us at 70 m/s despite the current of space dragging it away at 34 m/s.
I’m curious myself if I got that right. 
ETA: I composed this before I saw The Hamster King’s post… sorry for the redundant info.
And if my understanding is correct, this expansion is accumulative, in that at 1 million parsecs distant from us, objects are moving away from us at 45 mi/s due to space expanding. So, at 2 million parsecs away would this make objects appear to be moving at 90mi/s (and so on)?
Correct.
Would it be possible to give an image or a metaphor to show this? This is very counterintuitive to me. If the universe is flat, then I can’t imagine there are any lines that loop back on themselves.
On some of the more abstract ideas of space, time and the universe itself, are many of the theories “proven” true because the math works out?
Chronos said":
I don’t understand how space could ever be considered “finite”. To have a cosmic “wall” that no energy/light/matter can pass makes no sense. Maybe it’s mathematically possible, but its not intuitive at all.
But none of this is. If the universe is expanding, it must be expanding into something, right?
Sorry, but no. The universe is all there is. It expands by making itself larger. It doesn’t expand into anything.
Let’s try the two-dimensional analogy again. Take the surface of a balloon. Just the surface. There is no inside and no outside. Just a surface covered with dots. That’s everything, all there is. Now expand it. The balloon gets bigger. The dots get farther apart. Yet it didn’t expand into anything because there’s no outside; it just expanded.
The universe does the same thing in three dimensions. We can’t visualize down to it from a fourth dimension so that’s much harder to cope with. But it’s exactly the same.
I don’t understand how the surface of the Earth can be finite. To have an edge you can fall over makes no sense.
It’s basically the same thing.
The usual metaphor is various video-game worlds like Asteroids, where the map is flat, but the left edge of the map is identified with the right edge, and the top is identified with the bottom.
Every show that described Big Bang and universe’s inflation/expansion always did mention that and usually used the “we’re on the surface of an expanding balloon” analogy. I think a lot of people ignore that analogy for the fun visual of a big 'splosion.
I think the consensus for those who subscribe to the “conventional” Big Bang is that the initial “point” was Planck-scale small - that is, about 1.6×10^−35 meters. Of course, there are other theories of universe beginnings, for example involving two “branes” collision, that are “gentler” than the conventional Big Bang theory.
I don’t think you’re talking about a black hole there. Black holes cannot “spew out” anything by definition (except Hawking radiation which is miniscule for any reasonably-sized black hole).
There are some very good books out there. Dozens of them. I would suggest John Gribbin’s books - specifically “In Search of the Multiverse” and “In Search of Schrödinger’s Cat” - the second one, although almost 30 years old, is a very good intro to Quantum Mechanics.
The trouble is, the universe is expanding, and even if it is finite it is really really really big. So even if it were curved, given the size of the universe you might never hit yourself in the back of the head. Go far enough and the universe will be expanding away from us at more than 1 light year per year, and so you could never make it back to your starting point. Most of the universe will be forever outside our light cone–that is, we could never reach it even traveling at the speed of light.
So to use the numbers given above, the universe is expanding at 74,000 meters/second per million parsecs. Speed of light is about 300,000,000 meters/second. So according to my calculations, 4 billion parsecs away the universe is expanding away from us faster than light. That’s 13 billion light years. Huh. Any significance to that number?
No. If (as seems to be the case, to the best of our ability to determine) the Universe is infinite, then it has always been infinite.
In order of magnitude, yes. 13 billion years (which is called the Hubble time) is approximately the age of the Universe. The actual age can in principle be somewhat larger or smaller than the Hubble time, depending on the exact model used.
http://www.nasa.gov/vision/universe/starsgalaxies/wmap_pol.html
Inflation poses that the universe expanded far faster than the speed of light and grew from a subatomic size to a golf-ball size almost instantaneously.
It’s important to note that all the matter and energy we see today is not the same as all the matter and energy there is. The whole universe wasn’t compressed to a small region–the universe as a whole was extremely dense at the big bang, but not extremely small. The expansion after the bang was a rapid decrease in density, but not a rapid increase in size.
I learned that on the dope.
By “the universe” there they’re speaking loosely of the visible universe.
No, they aren’t. They specifically are talking about expansion that proceeds faster than the speed of light - thus creating regions in the universe that are far far away from the “visible universe”.
Here’s another one: Big Bang models back to Planck time
Then the universe was less than a billionth of a billionth of a billionth of a second old, it briefly went through a period of superchanged expansion, ballooning from the size of a proton to the size of a grapefruit (and thus expanding at many, many times the speed of light).
I had read that someone tried to calculate the size of the universe, and showed it was possible that the size of the universe is actually smaller (slightly) than the size of the visible universe—we just don’t recognize we’re looking at the backs of our heads, so to speak. Still plausible, or has new data come in to refine this calculation away from such a weird but neat conclusion?
I think a lot of people fail to understand that the universe is represented by just the surface of the balloon, and not the whole balloon (including the gas inside). I know that’s what tripped me up the first time I heard the analogy, and I understood a lot more after I grasped that concept.
As far as I know from reading the comments of physicists and other knowledgable people here on the dope, if the universe is flat, then the expansion of the universe consists in its constantly decreasing density, and not an increase in size. On the assumption that that is correct, I read all comments such as those you linked to as loosely referring to the visible universe using the phrase “the universe.” If anyone here with expertise knows better, though, I’m glad to be corrected.